DWDM optical communications systems reached the level of being able to successfully transmit and receive 10 Gbits/s per wavelength channel in the late 1990's. In continuing to strive for even greater capacity designers of optical communication systems have had to become inventive about how to increase the spectral efficiency. There are a limited number of channels that can be used in the well-known C-band (1530 nm to 1560 nm) and L-band (1560 nm to 1610 nm) wavelength ranges. As more channels are packed into the useable wavelength range of the optical communications spectrum the constraints of optical components in the communication system need to become more stringent. Laser transmitters and optical bandpass filters need tighter controls and thus become more expensive to produce. Achieving higher transmission rates using more channels also means that additional infrastructure may be required if attempting to use a pre-existing optical network.
A physical property of light is that it can exist in two distinct linear polarization states. The two linear polarization states are orthogonal with respect to each other. Taking advantage of this property a single wavelength can be used to carry two signals, a first signal being modulated on a first linear polarization state and a second signal being modulated on a second linear polarization state, which is orthogonal to the first linear polarization state.
Quadrature carrier modulation has been used extensively in radio frequency communication systems for many years. By modulating a portion of a signal with a local oscillator (LO) and modulating another portion of the signal with the same LO 90 degrees out of phase, it is possible to double the transmission spectral efficiency while using the same frequency band.
Taking advantage of the dual-polarization property of light and quadrature carrier modulation it is possible to achieve a four fold improvement in transmission efficiency while using the same frequency band. As a result, conventional 10 Gbit/sec long haul and ultra-long haul communication networks are capable of achieving 40 Gbits/sec without the extra monetary expense caused by tighter specifications on the optical components of the communication system or significant upgrades to existing infrastructure.
With the four fold improvement in transmission efficiency also comes increased difficulty in recovering transmitted signals at a receiver of a quadrature carrier dual-polarization optical communication system.
An optical fiber transmission channel introduces various forms of signal degradation that make it difficult to recover an original transmitted signal. Chromatic dispersion, polarization rotation, polarization mode dispersion (PMD) and polarization dependent loss (PDL) are typical factors that degrade the transmitted signal.
The transmitter and receiver components in the quadrature carrier dual-polarization optical communication system also introduce further signal degradation. Factors such as bandwidth limiting effects caused by inter-symbol-interference (ISI), imperfections in transmitter and receiver hardware, and phase noise generated by the transmitter laser and the receiver LO laser all degrade the transmitted signal.
In a quadrature carrier modulation scheme if two signals are not orthogonal to each other then there exists a phase angle error that causes signal interference to occur between the two signals making it impossible to recover either signal accurately. The phase angle error is commonly referred to as a quadrature angle error.
Quadrature orthogonality of received signals in the quadrature carrier dual-polarization optical communication system can be disrupted by imperfections in the transmitter hardware and the receiver hardware.